Certain manufacturing processes require heating and evaporating metals. For example, deposition of thin film of copper may be performed by evaporating copper at temperatures of 1300° C.-1600° C. These high temperatures are needed in order to achieve sufficiently high vapor pressure to enable commercially acceptable deposition rates. However, these high temperatures also drive taxing requirements on the crucibles used for the evaporation. Such crucibles are therefore generally made from refractory materials.
Typically BCC (body centered cubic) refractory metals (e.g., tungsten and tantalum) are used as heating elements. However, these materials have several significant drawbacks. In addition to being expensive, they become very brittle at high temperatures, leading to premature failure. Moreover, while bcc metals have relatively low resistivity at room temperature, their resistivity increases at high temperature, thereby placing stringent requirements on the power supplier design and costs.
Graphite rods and machined graphite elements have also been used as heating elements for evaporators. Such elements typically have low resistivity, thereby requiring high currents at low voltages, which also drives the power supply and delivery costs. While the resistivity of machined graphite elements can be increased, they are expensive, fragile, and difficult to machine into shapes which work with complex crucible geometry.
In view of the above, a solution is needed for a heating element and crucible arrangement having reduced costs, high reliability, and easy manufacturability.